The desirability of disk file units which can readily be removed and replaced or interchanged with other units has long been recognized.
However, some liabilities have occurred in the prior art in attempting to achieve this goal in that expensive and unwieldy "base units" were often required which would mechanically interconnect to the disk module in order to provide a driving force (for the disk pack and head carriage) through a shaft or a belt, at the same time attempting to seal off any contamination from the outside air or environment. This was typically the case in the "Winchester" disk unit as described in U.S. Pat. No. 3,843,967. These type of disk units, further, were not readily adaptable for use with the power frequencies of foreign countries (such as 50 Hz.) nor aircraft power at 400 Hz.
Other prior art attempts have been made in this area by providing sealed enclosures with transducers having head-per-track configurations. These usually proved to be extensively expensive and uneconomical.
The use of high-capacity random-access disk drives which permit operator interchangeable disk packs has usually been burdened with excessive costs. The interchangeability costs include additional control electronics and hardware for disk pack loading-and-unloading, for stringent contamination protection, for actuator carriage retraction, for head unloading, and adding mechanical couplings for spindle and head carriage driving. In order to offset these high costs, prior art disk drives were designed for very high capacity using many stacked disks and required relatively large drive motors having high power consumption; thus they were not really economically usable by a medium-to-small scale storage system.
One of the major costs of designing-in disk pack interchangeability (while keeping medium to low capacity storage) lies primarily in the alignment of the disk unit's transducing Read/Write heads, which must fly precisely relative to the disk surface. The head-to-disk track alignment must be sufficiently accomplished so that a disk unit, once written, may be removed and subsequently read on any of the other disk drive units. Thus, radial alignment of the transducing head with respect to the record track on the disk must be within 20% of the track width. The radial alignment tolerances must include static tolerances associated with head alignment differences between different drives, as well as dynamic tolerances involved, such as spindle-bearing runout, disk axial runout, actuator carriage bearing runout, thermal drift, vibration, and position accuracy of the access servo system.
Contamination during a disk pack exchange can have a serious effect on the reliability of a disk drive. When the disk pack module is removed from its "base drive", then the disk chamber, the disk pack mounting cone, and the transducing Read/Write heads (which are the most sensitive parts of the drive) are completely exposed to the environment.
Since the transducing heads often fly as close as 25 microinches to the disk surface, particles not visible to the naked eye, can cause disastrous failures.
In order to overcome some of the above problems and limitations, certain manufacturers built the "Winchester" disk cartridge, where disk cartridges were made to contain certain critical components (spindle, disk, Read/Write transducing heads, and an "actuator carriage") in an enclosed package.
This type of packaging somewhat reduced the costly positioning requirements for radial alignment and also facilitated greater track and bit density than was previously possible. Also at the same time, the reliability was greatly enhanced. However, since the "Winchester" disk cartridges did not contain the actuator voice-coil motor nor the spindle-motor, it was necessary that additional complex hardware be added to each disk cartridge, and each cartridge had to be supplied with a "base drive" in order to automatically couple the cartridges' "actuator carriage" to the voice-coil motor and also to couple the internal spindle to an external motor.
Besides being costly, these external-internal couplings introduced adverse reliability factors due to tolerance requirements, to possible mechanical failure; they also prevented complete sealing of the disk cartridge itself.
Further the "base drive" units (which were required for each disk module) had a cost factor of generally six times the cost of the disk module. Thus, if the disk module cost $1500.00, the base drive would generally cost at least $9000.00.